Powered devices need to have a mechanism to supply power to the operative parts. Typically systems use a physical power cable to transfer energy over a distance. There has been a continuing need for systems that can transmit power efficiently over a distance without physical structures bridging the physical gap.
Systems and methods that supply power without electrical wiring are sometimes referred to as wireless energy transmission (WET). Wireless energy transmission greatly expands the types of applications for electrically powered devices. One such example is the field of implantable medical devices. Implantable medical devices typically require an internal power source able to supply adequate power for the reasonable lifetime of the device or an electrical cable that traverses the skin. Typically an internal power source (e.g. battery) is feasibly for only low power devices like sensors. Likewise, a transcutaneous power cable significantly affects quality of life (QoL), infection risk, and product life, among many drawbacks.
More recently there has been an emphasis on systems that supply power to an implanted device without using transcutaneous wiring. This is sometimes referred to as a Transcutaneous Energy Transfer System (TETS). Frequently energy transfer is accomplished using two magnetically coupled coils set up like a transformer so power is transferred magnetically across the skin. Conventional systems are relatively sensitive to variations in position and alignment of the coils. In order to provide constant and adequate power, the two coils need to be physically close together and well aligned.
To operate efficiently, the transmit and receive resonators need to have a very low resistance, resulting in a very large quality factor (Q). Resonators with a large Q have a very narrow frequency band. To couple resonators with a large Q, the transmitter and receiver need to operate at precise frequencies. If a resonator has a Q of 100, then a 1% variation of a capacitor will dramatically de-tune the circuit. Since it is difficult to buy capacitors with a rated tolerance less than 5%, methods of trimming the capacitor value are required for efficient operation of wireless power systems.
Typically, prior systems have resorted to measuring capacitor values when the circuit is being built to find a capacitor that has a value within the desired range. These capacitors can be swapped out until a capacitor with the right value is found, which makes for a tedious and expensive circuit manufacturing process. Alternatively, several smaller capacitors can be combined, and the values of the smaller capacitors are measured and selected until the combined result is correct.